This invention relates to hydrocarbon trap/catalysts that are effective for trapping hydrocarbons and oxidizing the trapped hydrocarbons in an exhaust gas. The present invention is directed to enhancing oxidation of adsorbed hydrocarbons when the hydrocarbon trap/catalyst reaches a temperature at which the trapped (adsorbed) hydrocarbons are released.
Regulatory agencies have promulgated strict controls on the amounts of carbon monoxide, hydrocarbons and nitrogen oxides which automobiles can emit. The implementation of these controls has resulted in the use of catalytic converters to reduce the amount of pollutants emitted from automobiles.
To improve the emissions performance achievable by conversion catalyst compositions, particularly during cold-start operation, it has been proposed to use an adsorbent material to adsorb hydrocarbons during the cold-start period of engine operation. A number of patents disclose the broad concept of using an adsorbent material to minimize hydrocarbon emissions during cold-start engine operation. For example, U.S. Pat. No. 3,699,683 discloses an adsorbent bed placed after both a reducing catalyst and an oxidizing catalyst. That patent also discloses that when the exhaust gas stream is below 200xc2x0 C., the gas stream is directed through the reducing catalyst then through the oxidizing catalyst and finally through the adsorbent bed, thereby adsorbing hydrocarbons on the adsorbent bed. When the temperature goes above 200xc2x0 C. the gas stream which is discharged from the oxidation catalyst is divided into a major and minor portion. The major portion is discharged directly into the atmosphere. The minor portion is passed through the adsorbent bed, whereby unburned hydrocarbons are desorbed, and the resulting minor portion containing the desorbed unburned hydrocarbons is then passed into the engine where the desorbed unburned hydrocarbons are burned.
Another patent disclosing the use of both an adsorbent material and a catalyst composition to treat an automobile engine exhaust stream, especially during the cold-start period of engine operation, is U.S. Pat. No. 5,078,979. The adsorbent is a particular type of molecular sieve and the catalyst material which may be dispersed in the adsorbent may be a platinum group metal.
An international application published under the Patent Cooperation Treaty, International Publication Number WO 97/22404, discloses the use of an ion exchange reaction to alter the adsorption characteristics of a zeolite, thereby forming a basic zeolite said to be useful for adsorbing hydrocarbons from-exhaust streams. The ion exchange reaction takes place by mixing an alkaline metal or alkaline earth metal salt (sodium, calcium and magnesium are specifically disclosed) in an aqueous solution with the zeolite for a sufficient time and temperature to cause ion exchange. Typical reaction times range from 0.5 to 4.0 hours at from ambient up to 100xc2x0 C. and more typically 50 to 75xc2x0 C. The exchanged zeolite is then filtered and washed with water and dried. According to WO 97/22404, the basic zeolite can be formed into a slurry and then coated on to a carrier substrate.
In a publication by Mark G. Stevens and Henry C. Foley (Alkali Metals on Nanoporous Carbon: New Solid-Base Catalysts, Chem. Commun., 519-520 (1997)), it is disclosed that cesium may be entrapped in a carbogenic molecular sieve by vapor-phase deposition. In another publication by Stevens et al., (Mark G. Stevens, Keith M. Sellers, Shekhar Subramoney and Henry C. Foley, Catalytic Benzene Coupling on Caesium/Nanoporous Carbon Catalysts, Chem. Commun., 2679-2680 (1998)), such cesium entrapped carbogenic molecular sieves are said to have a high affinity for hydrogen, and for breaking of the Cxe2x80x94H bond in benzene and thereby promoting benzene condensation to a biphenyl.
Notwithstanding the foregoing, there remains a need for an improved hydrocarbon trap/catalyst for automotive cold-start operation emissions.
The present invention pertains to a hydrocarbon trap/catalyst, i.e. a hydrocarbon adsorbing material in which hydrocarbons are adsorbed at a low exhaust gas temperature characteristic of an engine start-up condition. In accordance with the present invention, this material is impregnated with an active metal to enhance oxidation of the hydrocarbons as the hydrocarbons are desorbed from the material at an elevated temperature characteristic of normal engine exhaust conditions. The invention optionally further comprises one or more layers of a support material impregnated with one or more platinum group metal catalysts, in combination with the impregnated hydrocarbon trap catalyst of the present invention.
The present invention differs from prior hydrocarbon trap/catalysts materials by providing an active metal deposited on and in intimate contact with the hydrocarbon adsorbent material but with little or no chemical reaction between the active metal and the adsorbent. While enhancing oxidation of hydrocarbons, this active metal does not affect the adsorption characteristics of the material.
The composition of the present invention typically comprises (a), as the hydrocarbon adsorbent material, a zeolite which is effective for adsorbing hydrocarbons from an engine exhaust and (b) an active metal in intimate contact with the zeolite. The invention optionally further comprises one or more layers of (c) a three way or oxidation catalyst that includes at least one platinum group metal (PGM) and preferably includes a combination of platinum group metals. Most preferably that combination comprises platinum, palladium and rhodium in a weight ratio collectively of about 12:5:1.
The active metal useful for the above purposes is essentially any alkaline metal or alkaline earth metal, such as potassium, rubidium, cesium, beryllium, magnesium, calcium, barium and strontium. Cesium is preferred. Generally, the active metal is deposited in intimate contact with the hydrocarbon adsorbent (zeolite, for example) by pouring, dipping or spraying a soluble salt solution of the active metal onto the adsorbent, which is then heated to dryness.
Optionally, the hydrocarbon adsorbent may be first deposited (prior to impregnation with the active metal) on a catalyst substrate, such as an inert monolithic or foam structure or inert pellets or beads.
While zeolite is the preferred hydrocarbon adsorbent in the present invention, other hydrocarbon adsorbents may also be useful. Among such possibilities are amorphous silica and certain forms of carbon or activated carbon, particularly including refractory forms of carbon such as Cn fullerenes.
The present invention comprises an improved hydrocarbon trap composition including a hydrocarbon-adsorbing material, such as zeolite, which is impregnated with an active metal, such that oxidation of desorbed hydrocarbons is enhanced. Such impregnation may be effected by contacting a dry zeolite with a soluble salt solution, such as an acetate or a nitrate of an active metal, namely an alkaline metal or alkaline earth metal, particularly cesium, and drying the wet zeolite with heating to remove water, leaving the metal in intimate contact with the zeolite but avoiding ion exchange therewith. A similar effect may be produced by slurrying zeolite alone in water and depositing the slurry on a monolithic catalyst substrate, drying the slurry to leave the zeolite in intimate contact with the substrate and then dipping, pouring or spraying an active metal solution over the zeolite substrate and drying that solution, with heat, as above, to leave active metal in intimate contact with the zeolite on the substrate.
Both natural and synthetic zeolites as well as acidic, basic or neutral zeolites may be used as the hydrocarbon adsorbent. Natural zeolites include faujasites, clinoptilolites, mordenites, and chabazites. Synthetic zeolites include ZSM-5, beta, Y, ultrastable-Y, mordenite, ferrierite, and MCM-22, with ZSM-5 and beta preferred. The SiO2:Al2O3 ratio for these materials is typically in the range of 2-1000, with a preferred SiO2:Al2O3 ratio of 30-300.
In use, the active metal acts as a catalyst for breaking Cxe2x80x94H bonds in the hydrocarbons. As indicated above, suitable active metals, which are believed to be useful for this purpose, are alkaline metals and alkaline earth metals, such as lithium, sodium, potassium, rubidium, cesium, beryllium, magnesium, calcium, barium and strontium. Of these, cesium is preferred.
Typically, the active metal-impregnated zeolite is a coating on a ceramic or metallic monolithic catalyst support or substrate which serves as an inert carrier for the active metal-containing zeolite and any subsequent catalyst coatings. Such support substrates may be porous or non-porous.
Following is a somewhat generalized but exemplary procedure for making an active metal impregnated zeolite in accordance with the present invention:
(a) Blending a zeolite and de-ionized water to form a slurry. 1 kilogram of zeolite is mixed with 2 liters of water.
(b) Adding to the slurry a binding material while the slurry is blending. The binder material is typically alumina or colloidal silicon dioxide. The binder is typically added in an amount of about 10-25% of the total weight of the zeolite to form a semi-solid mixture. The mixture is milled to obtain a nominal particle size of 1.0-20.0 microns, typically 4.5-5.0 microns. Once the desired particle size is achieved, the mixture is heated in flowing air at a temperature of in the range of 400xc2x0 C.-600xc2x0 C., typically about 500xc2x0 C., for 30-90 minutes, typically 30-60 minutes, until it is essentially dry. Alternatively, the zeolite slurry, prepared as above, may be applied to a support substrate, such as a monolith catalyst base of the type used in automotive exhaust systems, by pouring or spraying the zeolite slurry onto the support substrate or by dipping the support substrate into the zeolite slurry to form a first layer coating. If applied to a support substrate, the zeolite and support substrate are heated, as described above, until the zeolite firmly adheres to the inert carrier and any excess water has been evaporated. The amount of zeolite and binding material deposited should be at least 0.2, and preferably more than 1.0, but possibly as much as 4.0 g/in3 (g per 16.4 cm3).
(c) Impregnating at least one active metal onto the surface of the zeolite. Typically impregnation is accomplished at room temperature followed by drying at an elevated temperature. A solution of the active metal (e.g. an aqueous solution of CsNO3, CsC2H3O2 or some other soluble form of the active metal; 0.64 M CsNO3 in the example below) is poured over or sprayed on the zeolite. Alternatively, the zeolite or substrate on which zeolite has been deposited may be dipped into the solution. In any event, the zeolite is saturated with the active metal solution, by repeated contact steps if necessary, until enough of the solution has been absorbed in the zeolite to deposit a calculated amount of the active metal, taking into account the amount of solution absorbed and the concentration of the salt in the original solution. The amount of metal deposited should be at least 0.19, and preferably more than 3.7, but possibly as much as 16.2 weight % of active metal on the zeolite. The actual contact time to achieve this impregnation may be relatively short, on the order of 0.1 to 5 minutes, but generally is in the range of 0.5 to 2 minutes. One half minute of contact time is typically sufficient.
(d) Drying the active metal wet zeolite mixture. Typically the active metal solution/zeolite mixture is heated in flowing air at a temperature of about 400xc2x0 C.-600xc2x0 C., with 500xc2x0 C. particularly preferred, for 30-90 minutes, with 30-60 minutes preferred. In this manner, the zeolite and active metal solution mixture is heated to dryness, whereupon some or all of the metal in the active metal salt (typically a nitrate or acetate) is decomposed into either its metallic state or to a metal compound which is in intimate contact with the zeolite. Because the original impregnation occurred at room temperature and the subsequent heating occurred with relatively little water present, relatively little chemical interaction occurs between the active metal and the zeolite adsorbent in this impregnation process.
In a typical embodiment of the present invention, the active metal-impregnated zeolite is formed on a monolithic catalyst substrate and forms a first coating of a multi-layer catalyst structure. The overall composition of one such catalyst structure is described below. In forming such a structure, the second and succeeding catalyst layers may be produced in accordance with the invention disclosed in U.S. Pat. No. 6,022,825xe2x80x94Andersen et al. (the ""825 patent), of common assignment herewith, the entirety of which is incorporated herein by reference. For purposes of the present invention, the zirconium stabilized ceria of the second.layer, as disclosed in the ""825 patent, may be replaced with zirconium stabilized manganate, containing 20-70% zirconium oxide, and typically containing 40-65% zirconium oxide.
An optional third catalyst layer useful in the present invention comprises a washcoat which is also derived from one or more component slurries. This third catalyst layer, which when in combination with the active metal-impregnated zeolite of the present invention, enhances oxidation of hydrocarbons may be produced in accordance with the invention disclosed in PCT application WO 99/67020, also of common assignment herewith and also incorporated herein by reference.
Preferably, the optional third layer, together with the optional second layer (i.e. all catalyst layers combined with a hydrocarbon adsorbent trap) comprises, as the PGM constituents, platinum, palladium and rhodium in a weight ratio on the order of 12:5:1.